Freight transport is important to all industries relying on the production of, and subsequent sale, of physical products. Thus, there is a constantly increasing need not only for efficient, but also safe freight transport. Both legal requirements and common sense dictate that cargo carried on vehicles are secured to protect the people involved in loading, unloading and driving the vehicle in question, the load itself and the vehicle. Cargo securing may be obtained in different ways, but basic principles would include securing the cargo in such a way that it cannot shove away, roll-over, wander due to vibrations, fall off the vehicle, or make the vehicle tip over.
Normally several method(s) will be considered when securing cargo, including locking, blocking, direct lashing, top-over lashing or combinations of these methods.
However, regardless of what general method(s) is (are) used, also the use of equipment, which further supports the cargo securing such as friction sheets/mats, walking boards and/or edge beams. is necessary in most circumstances.
In relation to the tendency of cargo to slide during transport the so-called frictional force, i.e. the force exerted between an object and a surface when the object moves across the surface or is influenced by forces resulting in an effort to move the object across the surface, is central.
The frictional force can be expressed asFf=μFn 
Where, Ff=frictional force (N), μ=coefficient of friction, and Fn=normal force (N). For a cargo object (i.e. goods), which slides or is pulled/pushed horizontally, the normal force—Fn—is proportional to the weight:Fn=mg 
Where, m=mass of the cargo object (kg) and g=gravitational constant.
The causes of friction, and thus the applicable static frictional coefficient (μs) and dynamic frictional coefficient (μd), may in principle be subdivided into three phenomena, namely molecular adhesion, surface roughness, and plowing.
Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects along each other requires breaking these adhesive bonds. Thus, when two objects are brought into contact, many atoms or molecules from one object are in so close proximity to those in the other object that molecular or electromagnetic forces attract the molecules of the two materials together. Trying to slide one object across the other requires breaking these adhesive bonds. Some materials may have a composition that greatly increases their adhesion and making them “sticky” to touch.
Surface roughness is a factor which contributes when the materials are rough enough to cause serious abrasion. All solid materials have some degree of surface roughness. If the surfaces of two hard solids are extremely rough, the high points or asperities can interfere with sliding and cause friction because of the abrasion or wear that can take place when you slide one object against the other. This may be viewed as a “sandpaper effect”, where particles of the materials are dislodged from their surfaces. In such a case, friction is partly caused by surface roughness, although the adhesion effect still plays a part.
When one or more of the materials is relatively soft, much of the resistance to movement is caused by deformations of one of the objects or by a plowing effect. Soft materials will deform when under pressure. This also increases the resistance to motion. When materials deform, the harder material must “plow” through the softer material to move, thus creating a resistive force. When the deformation becomes large, such that one object sinks into the other, this can affect the friction.
During transport the static frictional forces resulting from the interaction of two surfaces, e.g. the surface of a vehicle and the surface of the cargo objects, will as a starting point act to prevent any relative motion of the relevant objects. The threshold of motion is characterized by the static frictional coefficient (μs). Static friction resistance will normally match the applied force up until the threshold of motion. When two surfaces (e.g. the surface of a vehicle and the surface of the cargo) are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds, and is characterized by the so-called dynamic frictional coefficient (μd). In most cases it is easier to keep something in motion across a horizontal surface than it is to start it in motion from rest, which would indicate that the dynamic frictional coefficient (μd) is less than or equal to the static frictional coefficient (μs). On the other hand, the friction to be overcome to start motion (static friction) will in some scenarios (e.g. dry metals) not significantly exceed the force required to maintain motion (dynamic friction).
Dynamic and static frictional coefficients may be determined using a frictometer, which may e.g. be constructed, using a load cell connected to an analog to digital converter, which is again connected to a PC in order to obtain the data through a custom made software application.
A possible design of a frictometer capable of measuring friction relevant for transportation situations is depicted in FIG. 3.
The period of time that two objects are in contact with one another prior to being subjected to lateral forces may also play a role in relation to the resulting static frictional coefficient. This is important because varying resting time of cargo may apply in real life situations before an acting pressure is in fact applied. Thus, a cargo object might be five days underway to its destination before the method of transportation causes the object to experience a high amount of lateral force, in such case the resting time might result in sufficient static friction for the object to remain stationary. Another object could have just left the production facility when the same amount of lateral force is applied, resulting in a static friction that is not high enough to avoid motion and thus damage.
Also, the contact pressure acting on the coating is important, as the viscoelastic properties of a coating is expected to result in a decrease in static friction coefficient with increased contact pressure.
Contact between different materials will give rise to different coefficients of friction. Table 1 below shows standard values for static frictional coefficients, which are valid provided that both contact surfaces are dry, clean and free from e.g. frost, ice and snow.
TABLE 1MATERIAL COMBINATIONμsTIMBER/WOODSawn timber against plywood/plyfa/wood0.5Sawn timber against grooved aluminum0.4Sawn timber against steel metal0.4Sawn timber against shrink film0.3Brick against Wood0.6Cast against Iron Oak0.5Hemp rope against Timber0.5Leather against Oak0.6Leather against Wood0.4Oak against Oak (parallel grain)0.6Oak against Oak (cross grain)0.5Wood against Concrete0.6SHRINK FILM/NYLON/POLYSTYRENE/RUBBERShrink film against plyfa0.3Shrink film against grooved aluminum0.3Shrink film against steel metal0.3Shrink film against shrink film0.3Nylon against Nylon0.2Polystyrene Polystyrene0.5Rubber against Rubber1.2PAPER CARDBOARD (UNTREATED)Cardboard against cardboard0.5Cardboard against wooden pallet0.5Rubber against Cardboard0.8BIG BAGBig bag against wooden pallet0.4STEEL AND SHEET METALFlat steel against sawn timber0.5Unpainted rough sheet metal against sawn timber0.5Painted rough sheet metal against sawn timber0.5Unpainted rough sheet metal against unpainted rough sheet metal0.4Painted rough sheet metal against painted rough sheet metal0.3Painted metal barrel against painted metal barrel0.2Bronze against Steel0.2Carbon against Steel0.1Leather against Metal0.4Paper against Cast Iron0.2Plexiglas against Steel0.5Polystyrene against Steel0.4
As is evident from the values reported above most combinations, apart from those involving rubber, give rise to static frictional coefficients either at or below 0.6.